To obtain hydrocarbons such as oil and gas from subterranean formations, wellbores are drilled into the formations by rotating a drill bit attached to an end of a drill string. A substantial portion of current drilling activity involves what is referred to in the art as “directional” drilling. Directional drilling involves drilling deviated and/or horizontal wellbores (as opposed to straight, vertical wellbores). Modern directional drilling systems generally employ a bottom hole assembly at the end of the drill string that includes a drill bit and a hydraulically actuated motor to drive rotation of the drill bit. The drill bit is coupled to a drive shaft of the motor, and drilling fluid pumped through the motor (and to the drill bit) from the surface drives rotation of the drive shaft to which the drill bit is attached. Such hydraulic motors are commonly referred to in the drilling industry as “mud motors,” “drilling motors,” and “Moineau motors.” Such motors are referred to hereinafter as “hydraulic drilling motors.”
Hydraulic drilling motors include a power section that contains a stator and a rotor disposed in the stator. The stator may include a metal housing that is lined inside with a helically contoured or lobed elastomeric material. The rotor is usually made from a suitable metal, such as steel, and has an outer lobed surface. Pressurized drilling fluid (commonly referred to as drilling “mud”) is pumped into a progressive cavity formed between the rotor and the stator lobes. The force of the pressurized fluid pumped into and through the cavity causes the rotor to turn in a planetary-type motion. A suitable shaft connected to the rotor via a flexible coupling compensates for eccentric movement of the rotor. The shaft is coupled to a bearing assembly having a drive shaft (also referred to as a “drive sub”), which in turn rotates the drill bit attached thereto.
As drilling fluid flows through the progressive cavity between the rotor and the stator, the drilling fluid may erode surfaces of the rotor and/or the stator within the progressive cavity. Such erosion may be relatively more severe at locations at which the direction of fluid flow changes, since the drilling fluid may impinge on the surfaces at relatively higher angles at such locations. This erosion can eventually result in the deformation of the lobes of the rotor and/or the stator, which can adversely affect operation of the hydraulic drilling motor.